1. Field of the Invention
The invention relates to a system for measuring the engagement pressure between two cylinders of a printing press, more particularly the impression cylinder and the inking cylinder of a rotary press.
2. Background Art
To obtain high-quality and reproducible printing, the engagement pressure between the impression cylinder and inking cylinders in a rotary press must be accurately determined and maintained constant. While the press is running, the engagement pressure may change due to changes in the characteristics of the paper, changes in the temperature, and swelling of the blankets mounted on the inking cylinders.
Early attempts at measurement of the engagement pressure included the use of pressure measuring cells or elongation measuring strips such as piezoelectric pressure cells and strain gauges. These techniques are difficult and costly to employ and do not have the long-term stability needed for the continuous monitoring of pressure under actual running conditions.
A suitble device for the continuous monitoring of engagement pressure is described in Hartung U.S. Pat. No. 3,965,735 issued June 29, 1976. A first mirror is fixed to the end of a journal of the one of the cylinders in a position perpendicular to the cylinder axis, and a relatively stationary second mirror is spaced substantially parallel to the first mirror. A light emitter at one edge of the mirrors, and lying in the common plane of the cylinders, directs a concentrated beam of light between the mirrors at a shallow angle of incidence for multiple reflection, with the point of exit of the beam at the other edge of the mirrors shifting in accordance with changes in the angular position of the first mirror as a result of slight bowing of the cylinder in response to changes in the engagement pressure. A photocell at the region of exit receives the beam to produce an output signal which varies in accordance with the shift in the point of exit of the beam to constitute measure of the applied pressure. This known device, however, has the disadvantage that the mirror surfaces may become soiled and oiled up during operation of the press, so that the necessary accuracy of measurement cannot always be ensured.